Photosensor system and drive control method for optimal sensitivity

ABSTRACT

A photosensor system includes a photosensor array constituted by two dimensionally arraying a plurality of photosensors, an image reading section which reads a subject image at a predetermined image reading sensitivity by using the photosensor array, a sensitivity adjustment image reading section which reads a sensitivity adjustment subject image while the image reading sensitivity in the photosensor array is changed to a plurality of stages, an optimal image reading sensitivity deriving section which derives an image reading sensitivity optimal for reading operation of the subject image, on the basis of a pixel data group relating to an image pattern of the subject image read by the sensitivity adjustment image reading section, and an image reading sensitivity setting section which sets the optimal image reading sensitivity as the image reading sensitivity in the image reading section.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation Application of U.S.application Ser. No. 10/171,972 filed Jun. 14, 2002, which is based uponand claims the benefit of priority from the prior Japanese PatentApplications No. 2001-183623, filed Jun. 18, 2001; and No. 2002-042747,filed Feb. 20, 2002, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photosensor system having aphotosensor array constituted by two-dimensionally arraying a pluralityof photosensors, and a drive control method in the photosensor system.

2. Description of the Related Art

One of conventional two-dimensional image reading apparatuses forreading printed matter, a photograph, or a fine three-dimensional shapesuch as a fingerprint is a photosensor system having a photosensor arrayconstituted by arraying photoelectric converting elements (photosensors)arranged in a matrix.

As well known, a CCD has a structure in which photosensors such asphotodiodes or thin film transistors (TFT: Thin Film Transistor) arearranged in a matrix, and the amount of electron-hole pairs (the amountof charge) generated corresponding to the amount of light entering alight receiving section of each photosensor is detected by a horizontalscanning circuit and vertical scanning circuit to detect the luminanceof radiation.

In a photosensor system using such a CCD, it is necessary torespectively provide selective transistors for causing the scannedphotosensor to assume a selected state. This increases the system sizeas the number of pixels increases.

In place of the combination of the photosensor and the selectivetransistor, a photosensor (to be referred to as a double-gatephotosensor hereinafter) is now being developed, which is formed of athin film transistor having a so-called double-gate structure and hasboth a photosensing function and a selecting function.

FIG. 26A is a sectional view showing the structure of a double-gatephotosensor 10. FIG. 26B is a circuit diagram showing the equivalentcircuit of the double-gate photosensor 10.

The double-gate photosensor 10 comprises a semiconductor thin film 11formed of amorphous silicon or the like, n⁺-silicon layers 17 and 18respectively formed at the two ends of the semiconductor thin film 11,source and drain electrodes 12 and 13 respectively formed on then⁺-silicon layers 17 and 18, a top gate electrode 21 formed above thesemiconductor thin film 11 via a block insulating film 14 and upper gateinsulating film 15, a protective insulating film 20 provided on the topgate electrode 21, and a bottom gate electrode 22 provided below thesemiconductor thin film 11 via a lower gate insulating film 16. Thedouble-gate photosensor 10 is provided on a transparent insulatingsubstrate 19 formed of glass or the like.

In other words, the double-gate photosensor 10 includes an upper MOStransistor comprised of the semiconductor thin film 11, source electrode12, drain electrode 13, and top gate electrode 21, and a lower MOStransistor comprised of the semiconductor thin film 11, source electrode12, drain electrode 13, and bottom gate electrode 22. As is shown in theequivalent circuit of FIG. 26B, the double-gate photosensor 10 isconsidered to include two MOS transistors having a common channel regionformed of the semiconductor thin film 11, TG (Top Gate terminal), BG(Bottom Gate terminal), S (Source terminal), and D (Drain terminal).

The protective insulating film 20, top gate electrode 21, upper gateinsulating film 15, block insulating film 14, and lower gate insulatingfilm 16 are all formed of a material having a high transmittance ofvisible light for activating the semiconductor layer 11. Light enteringthe sensor from the top gate electrode 21 side passes through the topgate electrode 21, upper gate insulating film 15, and block insulatingfilm 14, and then enters the semiconductor thin film 11, therebygenerating and accumulating charges (positive holes) in a channel regiontherein.

FIG. 27 is a schematic view showing a photosensor system constituted bytwo-dimensionally arraying double-gate photosensors 10. As shown in FIG.27, the photosensor system comprises a sensor array 100 that isconstituted of a large number of double-gate photosensors 10 arranged inan n×m matrix, top and bottom gate lines 101 and 102 that respectivelyconnect the top gate terminals TG and bottom gate terminals BG of thedouble-gate photosensors 10 in a row direction, top and bottom gatedrivers 110 and 120 respectively connected to the top and bottom gatelines 101 and 102, data lines 103 that respectively connect the drainterminals D of the double-gate photosensors 10 in a column direction,and an output circuit section 130 connected to the data lines 103.

In FIG. 27, φtg and φbg represent control signals for generating a resetpulse φTi and readout pulse φBi, respectively, which will be describedlater, and φpg represents a pre-charge pulse for controlling the timingat which a pre-charge voltage Vpg is applied.

In the above-described structure, as described later, the photosensingfunction is realized by applying a predetermined voltage from the topgate driver 110 to the top gate terminals TG, while the readout functionis realized by applying a predetermined voltage from the bottom gatedriver 120 to the bottom gate terminals BG, then sending the outputvoltage of the photosensors 10 to the output circuit section 130 via thedata lines 103, and outputting serial data Vout.

FIGS. 28A to 28D are timing charts showing a drive control method of thephotosensor system, and showing a detecting period (i-th row processingcycle) in the i-th row of the sensor array 100. First, a high-levelpulse voltage (reset pulse; e.g., Vtgh=+15V) φTi shown in FIG. 28A isapplied to the top gate line 101 of the i-th row, and during a resetperiod T_(rest), reset operation for discharging the double-gatephotosensors 10 of the i-th row is executed.

Subsequently, a bias voltage φTi of low level (e.g., Vtgl=−15V) isapplied to the top gate line 101, thereby finishing the reset periodT_(rest) and starting a charge accumulating period Ta in which thechannel region is charged. During the charge accumulating period Ta,charges (positive holes) corresponding to the amount of light enteringeach sensor from the top gate electrode side are accumulated in thechannel region.

Then, a pre-charge pulse φpg shown in FIG. 28C with a pre-charge voltageVpg is applied to the data lines 103 during the charge accumulatingperiod Ta, and after a pre-charge period T_(prch) for making the drainelectrodes 13 keep a charge, a bias voltage (readout pulse φBi) of highlevel (e.g., Vbgh=+10V) shown in FIG. 28B is applied to the bottom gateline 102. At this time, the double-gate photosensors 10 are turned on tostart a readout period T_(read).

During the readout period T_(read), the charges accumulated in thechannel region serve to moderate a low-level voltage (e.g., Vtgl=−15V)which has an opposite polarity of charges accumulated in the channelregion and is applied to each top gate terminal TG. Therefore, an n-typechannel is formed by the voltage Vbgh at each bottom gate terminal BG,the voltage VD at the data lines 103 gradually reduces in accordancewith the drain current with lapse of time after the pre-charge voltageVpg is applied. More specifically, the tendency of change in the voltageVD at the data lines 103 depends upon the charges accumulating period Taand the amount of received light. As shown in FIG. 28D, the voltage VDtends to gradually reduce when the incident light is dark, i.e., a smallamount of light is received, and hence only small charges areaccumulated, whereas the voltage VD tends to suddenly reduce when theincident light is bright, i.e., a large amount of light is received, andhence large charges are accumulated. From this, it is understood thatthe amount of radiation can be calculated by detecting the voltage VD atthe data lines 103 a predetermined period after the start of the readoutperiod T_(read), or by detecting a period required until the voltage VDreaches a predetermined threshold voltage.

Image reading is performed by sequentially executing the above-describeddrive control for each row of the sensor array 100, or by executing thedrive control for each row in a parallel manner at different timings atwhich the driving pulses do not overlap.

Although the photosensor system adopts the double-gate photosensor as aphotosensor in the above description, even a photosensor system using aphotodiode or phototransistor as a photosensor has operation steps:reset operation→charge accumulating operation→pre-chargeoperation→reading operation, and uses a similar drive sequence. Theconventional photosensor system as above has the following problems.

In this photosensor system, such a photosensor array is formed on onesurface of the transparent substrate such as a glass substrate, asdescribed above, and a light source is provided on the back surface sideof the transparent substrate. Light emitted by the light sourceirradiates a subject (finger or the like) placed above the photosensorarray. The reflected light corresponding to the image pattern of afingerprint or the like is received and detected as brightnessinformation by each photosensor, reading the subject image. Imagereading operation of the photosensor array detects brightnessinformation on the basis of the amount of charges accumulated in eachphotosensor during a period corresponding to a set image readingsensitivity (charge accumulating period for the double-gatephotosensor).

In the photosensor system using the above-described photosensor, factorsincluding an environmental illuminance in a use place such as an indooror outdoor place and the type of subject change depending on a useenvironment. To read a subject image in various use environments, theimage reading sensitivity of the photosensor must be properly adjusted.

The proper image reading sensitivity of the photosensor changesdepending on ambient conditions such as an environmental illuminance. Inthe prior art, therefore, a circuit for detecting the environmentalilluminance must be additionally arranged. Alternatively, readingoperation is done for a standard sample placed on the sensing surfacebefore the start of normal image reading operation, while the readingsensitivity is changed to a plurality of values. An optimal imagereading sensitivity corresponding to ambient conditions such as theenvironmental illuminance is obtained and set on the basis of detectionresult or reading result. However, the above-described prior art suffersthe following problems.

(1) When the photosensor system is applied to a fingerprint readingapparatus or the like, the state of the skin surface layer of a finger(or human body) serving as a subject varies depending on the gender andage of the person, the individual difference such as the physicalcondition, or an external environment such as a temperature or humidity.This inhibits setting a proper image reading sensitivity when the imagereading sensitivity is set based on reading operation before the startof normal image reading operation. For this reason, the apparatusmalfunctions in fingerprint collation processing or the like.

More specifically, if the skin surface layer of the finger as a subjectis keratinized, the brightness of the ridge pattern of the keratinizedfingerprint is observed higher than that of a non-keratinized normalskin surface. The brightness difference detected by the photosensorbecomes larger than an original value. If the image reading sensitivityis set based on this brightness information, the image readingsensitivity is set to a lower value than an originally appropriatevalue. As a result, a subject image such as a fingerprint cannot beaccurately read, decreasing the collation precision of the fingerprint.

(2) If a foreign substance deposited on the sensing surface of thephotosensor or a defect is generated in the photosensor element inreading operation before normal image reading operation, the direct useof a read result containing an abnormal value causes a failure insetting a proper image reading sensitivity. This inhibits accuratereading operation of a subject image. When this photosensor system isapplied to a fingerprint reading apparatus, the apparatus maymalfunction in fingerprint collation processing.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an advantage to set a proper imagereading sensitivity regardless of the individual difference of a subjector an environment in a photosensor system having a photosensor arrayconstituted by two-dimensionally arraying a plurality of photosensors,and to set a proper image reading sensitivity and obtain a high-qualitysubject image by normal image reading operation even in the presence ofabnormal pixel data caused by a pixel defect or foreign matter.

To achieve the above advantages, a photosensor system according to thepresent invention comprises a photosensor array constituted bytwo-dimensionally arraying a plurality of photosensors, an image readingsection which reads a subject image at a predetermined image readingsensitivity by using the photosensor array, a sensitivity adjustmentimage reading section which reads a sensitivity adjustment subject imagewhile the image reading sensitivity in the photosensor array is changedto a plurality of stages, an optimal image reading sensitivity derivingsection which derives an image reading sensitivity optimal for readingoperation of the subject image on the basis of a pixel data grouprelating to an image pattern of the subject image read by thesensitivity adjustment image reading section, and an image readingsensitivity setting section which sets the optimal image readingsensitivity as the image reading sensitivity in the image readingsection.

To achieve the above advantages, the first optimal image readingsensitivity deriving section according to the present inventioncomprises a standard image reading sensitivity extraction section whichextracts as a standard image reading sensitivity an image readingsensitivity having a maximum data range of the pixel data group on thebasis of a pixel data group for each image reading sensitivity relatingto the image pattern of the subject image read by the sensitivityadjustment image reading section, an image reading sensitivitycorrection section which corrects the standard image reading sensitivityon the basis of a predetermined period-increase rate, and a sectionwhich sets a value obtained by correcting the standard image readingsensitivity as the optimal image reading sensitivity. The image readingsensitivity correction section corrects the standard image readingsensitivity uniquely or by the predetermined period-increase rate on thebasis of a result of comparing a mean value of the pixel data group atthe standard image reading sensitivity with a predetermined referencevalue.

To achieve the above advantages, the second optimal image readingsensitivity deriving section according to the present inventioncomprises a specific data removal section which removes specific pixeldata from the pixel data group for each image reading sensitivityrelating to the image pattern of the subject image read by thesensitivity adjustment image reading section, an image readingsensitivity extraction section which extracts an image readingsensitivity having a maximum data range of the pixel data group as animage reading sensitivity suitable for normal reading operation of thesubject image on the basis of the pixel data group for each imagereading sensitivity from which the specific pixel data is removed by thespecific data removal section, and a setting section which sets theimage reading sensitivity extracted by the image reading sensitivityextraction section as the optimal image reading sensitivity. Thespecific data removal section removes, from the pixel data groupobtained by the sensitivity adjustment reading section for each imagereading sensitivity, pixel data having a maximum or minimum value or aplurality of pixel data sequentially from the maximum or minimum value.

Even if the individual difference of a subject or the externalenvironment changes or pixel data contains an abnormal value, theseinfluences can be avoided to properly set the image reading sensitivityand obtain a high-quality subject image.

Additional advantages of the invention will be set forth in thedescription which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. Theadvantages of the invention may be realized and obtained by means of theinstrumentalities and combinations particularly pointed out hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 is a block diagram showing an arrangement of a photosensor systemaccording to a first embodiment of the present invention;

FIG. 2 is a block diagram showing an arrangement of a controller appliedto the first embodiment;

FIG. 3 is a flow chart showing an example of sensitivityadjustment/setting processing executed by the controller according tothe first embodiment;

FIG. 4 is a view showing an example of image data when a fingerprintimage is read in the first embodiment;

FIGS. 5A to 5E are graphs showing changes in lightness data ofrespective rows in fingerprint image data obtained by sensitivityadjustment reading operation in the first embodiment;

FIGS. 6A and 6B are tables showing an example of the relationshipbetween the dynamic range of lightness data for respective rows obtainedby sensitivity adjustment reading operation in the first embodiment, anda row number vs. image reading sensitivity correspondence table;

FIGS. 7A and 7B are a view showing a fingerprint image, and a graphshowing an example of changes in lightness data of respective columns atan image reading sensitivity when normal image reading operation isperformed at an image reading sensitivity obtained without executingperiod-increase correction processing in the first embodiment;

FIGS. 8A and 8B are a view showing a fingerprint image, and a graphshowing an example of changes in lightness data of respective columns atan image reading sensitivity when normal image reading operation isperformed at an image reading sensitivity obtained by executingperiod-increase correction processing in the first embodiment;

FIG. 9 is a block diagram showing an arrangement of a controller appliedto a second embodiment;

FIG. 10 is a flow chart showing an example of sensitivityadjustment/setting processing executed by the controller according tothe second embodiment;

FIG. 11 is a block diagram showing an arrangement of a photosensorsystem according to a third embodiment of the present invention;

FIG. 12 is a block diagram showing an arrangement of a controllerapplied to the third embodiment;

FIG. 13 is a flow chart showing an example of sensitivityadjustment/setting processing executed by the controller according tothe third embodiment;

FIGS. 14A and 14B are conceptual views showing a target region and anexample of reading operation in sensitivity adjustment reading operationaccording to the third embodiment;

FIGS. 15A and 15B are conceptual views showing another target region andanother example of reading operation in sensitivity adjustment readingoperation according to the third embodiment;

FIGS. 16A to 16D are conceptual views showing an example of a specificpixel data removal operation method applied to the third embodiment;

FIGS. 17A and 17B are views showing a predetermined detection region ofa photosensor array in the third embodiment, and an example of afingerprint image read within the detection region by sensitivityadjustment reading operation when an abnormal pixel exists;

FIGS. 18A to 18E are graphs showing an example of changes in lightnessdata for respective rows of fingerprint image data obtained bysensitivity adjustment reading operation in the third embodiment when anabnormal pixel exists;

FIGS. 19A and 19B are a graph and table showing an example of therelationship between the dynamic range of lightness data for respectiverows of fingerprint image data obtained by sensitivity adjustmentreading operation when an abnormal pixel exists, and a row number vs.image reading sensitivity correspondence table;

FIGS. 20A and 20B are views showing examples of the image pattern of afingerprint when a sensitivity setting method according to the thirdembodiment is not applied and is applied in setting an image readingsensitivity using a photosensor array including an abnormal pixel;

FIGS. 21A to 21E are graphs showing an example of changes in lightnessdata for respective rows of fingerprint image data obtained by specificpixel data removal operation in the third embodiment;

FIGS. 22A and 22B are a graph and table showing an example of therelationship between the dynamic range of lightness data for respectiverows obtained by performing specific pixel data removal operation in thethird embodiment for fingerprint image data which is obtained bysensitivity adjustment reading operation when an abnormal pixel exists,and a row number vs. image reading sensitivity correspondence table;

FIGS. 23A to 23J are timing charts showing an example of a drive controlmethod applicable to image reading operation of a photosensor systemaccording to the present invention;

FIGS. 24A to 24J are timing charts showing the first example of an imagereading sensitivity setting method applicable to sensitivity adjustmentreading operation in each embodiment;

FIGS. 25A to 25J are timing charts showing the second example of theimage reading sensitivity setting method applicable to sensitivityadjustment reading operation in each embodiment;

FIG. 26A is a sectional view showing the structure of a double-gatephotosensor;

FIG. 26B is a circuit diagram showing the equivalent circuit of thedouble-gate photosensor;

FIG. 27 is a schematic view showing a photosensor system constituted bytwo-dimensionally arraying double-gate photosensors; and

FIGS. 28A to 28D are timing charts showing the drive control method of adouble-gate photosensor system.

DETAILED DESCRIPTION OF THE INVENTION

Details of a photosensor system and drive control method thereofaccording to the present invention will be described on the basis ofembodiments shown in the several views of the accompanying drawing. Inembodiments described below, a double-gate photosensor is applied as aphotosensor. However, the arrangement of the present invention is notlimited to the double-gate photosensor, but is also applicable to aphotosensor system using another type of photosensor.

First Embodiment

A first embodiment of a photosensor system according to the presentinvention will be described with reference to the several views of theaccompanying drawings.

FIG. 1 is a block diagram showing an arrangement of a photosensor systemaccording to the first embodiment. The double-gate photosensor shown inFIG. 26A is employed as a photosensor, and the arrangement of thephotosensor system shown in FIG. 27 will be referred to if necessary.The same reference numerals as in the photosensor system shown in FIG.27 denote the same parts, and a description thereof will be simplifiedor omitted.

As is shown in FIG. 1, the photosensor system according to thisembodiment comprises a photosensor array 100 including double-gatephotosensors 10 shown in FIG. 26A that are arrayed two-dimensionally, atop gate driver 110 for applying a predetermined reset pulse to a topgate TG (FIG. 27) of each double-gate photosensor 10 at a predeterminedtiming, a bottom gate driver 120 for applying a predetermined readoutpulse to a bottom gate BG of the double-gate photosensor 10 at apredetermined timing, an output circuit section 130 which has a columnswitch 131, pre-charge switch 132, and amplifier 133, applies apre-charge voltage to the double-gate photosensor 10, and reads out adata line voltage, an analog/digital converter (to be referred to as anA/D converter hereinafter) 140 for converting the data voltage of areadout analog signal into image data of a digital signal, a controller150 which is adopted to control the operation of reading a subject imageby the photosensor array 100, and to exchange data with an externalfunction section 200, and which controls sensitivity setting in thefirst embodiment, and a RAM 160 for temporarily storing acquired imagedata (pixel data group), and processing data or the like relating tosensitivity setting processing (to be described later).

The structures of the photosensor array 100, top gate driver 110, bottomgate driver 120, and output circuit section 130 are the same as and havethe same functions as these of the photosensor system shown in FIG. 27.In addition to these members, this embodiment adopts the A/D converter140, controller 150, and RAM 160 to enable various types of control asdescribed below.

The controller 150 outputs predetermined control signals φtg and φbg tothe top and bottom gate drivers 110 and 120, respectively, which, inturn, output predetermined voltages (reset pulse φTi and readout pulseφBi) to the top gates TG and bottom gates BG of the double-gatephotosensors 10 which constitute the photosensor array 100,respectively. The controller 150 also outputs a predetermined controlsignal φpg to the pre-charge switch 132 to apply a pre-charge voltageVpg to the drain terminals D of the double-gate photosensors 10. Thecontroller 150 controls an operation of detecting a drain voltage VDcorresponding to the amount of charges accumulated in the double-gatephotosensors 10 in correspondence with the image pattern of a readsubject. An output voltage V_(out) read out by the drain driver 130 isconverted into a digital signal via the A/D converter 140, and thisdigital signal is input as an image output signal to the controller 150.The controller 150 has a function of executing predetermined imageprocessing for the image output signal, and writing or reading theprocessed signal into or from the RAM 160. The controller 150 alsofunctions as an interface with an external function section 200 thatexecutes predetermined processing such as image data identification,modification, and the like.

The controller 150 has another function of changing and controllingcontrol signals to be output to the top and bottom gate drivers 110 and120 to set an optimal reading sensitivity for reading a subject image inaccordance with environments such as an illuminance of external light,i.e., an optimal charge accumulating period for the double-gatephotosensors 10.

The external function section 200 has a function of executing imageprocessing (collation, modification, or the like) for, e.g., image dataacquired by the photosensor system in accordance with the applicationpurpose of the photosensor system. The external function section 200also has a function of monitoring the operation state of the photosensorsystem or controller 150 and outputting a calculation processing resultor the like. Further, the external function section 200 also functionsas an input/output interface for inputting and setting variousparameters such as the default value of a charge accumulating period Ta,and a period-increase rate and mean compare value (to be describedlater) in order to define the operation state, calculation processing,and the like.

The detailed arrangement and operation of the controller applied to thephotosensor system described above will be explained with reference tothe several views of the accompanying drawings.

FIG. 2 is a block diagram showing an arrangement of the controllerapplied to the photosensor system according to the first embodiment.

As shown in FIG. 2, the controller 150 in this embodiment comprises adevice controller 151 for controlling the operations of the top gatedriver 110, bottom gate driver 120, and pre-charge switch 132, a datacontroller 152 for managing various data such as image data, write data,and readout data to the RAM 160, and a main controller 153 whichsupervises the controllers 151 and 152 and interfaces with the externalfunction section 200.

The controller 150 further comprises: a data comparator 154 forextracting maximum and minimum values by comparing the sizes oflightness data in pixel data contained in an image output signal on thebasis of image data input as a digital signal from the photosensor array100 via the A/D converter 140, and for extracting a maximum dynamicrange from calculated dynamic ranges; an adder 155 for calculating adynamic range (data range) from the difference between the maximum andminimum values of pixel data extracted by the data comparator 154; adata selector 156 for receiving image data and processing data processedvia the A/D converter 140, data comparator 154, and adder 155, andswitching between write/readout of these data in/from the RAM 160,re-input of these data to the data comparator 154 and adder 155, andoutput of these data to the external function section 200 via the datacontroller 152 if necessary; a period-increase register 157 which holdsa period-increase rate used to execute period-increase correction (to bedescribed later) by the main controller 153 for an image readingsensitivity which is extracted by the data comparator 154, adder 155,and data selector 156 and corresponds to a maximum dynamic range; and asensitivity setting register 159 for setting the timings of controlsignals φtg and φbg to be output from the device controller 151 to thetop and bottom gate drivers 110 and 120 on the basis of a control signaloutput from the data controller 152 based on the image readingsensitivity (optimal image reading sensitivity) having undergoneperiod-increase correction.

Processing operation by the above-mentioned controller will be explainedwith reference to the several views of the accompanying drawings.

FIG. 3 is a flow chart showing an example of sensitivityadjustment/setting processing executed by the controller applied to thephotosensor system according to the first embodiment. FIG. 4 is a viewshowing an example of fingerprint image data when the photosensor systemaccording to this embodiment is applied to a fingerprint readingapparatus and a fingerprint is read while the image reading sensitivityis changed stepwise every row of the photosensor array in sensitivityadjustment reading operation applied to sensitivity adjustmentprocessing. FIGS. 5A to 5E are graphs showing changes in lightness dataof respective rows in fingerprint image data obtained by sensitivityadjustment reading operation. FIGS. 6A and 6B are tables showing therelationship between the dynamic range (data range) of lightness datafor respective rows obtained by sensitivity adjustment readingoperation, and a row number vs. image reading sensitivity correspondencetable. The processing operation will be explained with reference to thearrangement of the photosensor system shown in FIGS. 1 and 2. As forvarious data such as lightness data, its dynamic range, and the chargeaccumulating period, only data of representative 80th, 104th, 128th,152nd, and 176th rows are illustrated.

(Step S11)

As shown in FIG. 3, the main controller 153 starts sensitivityadjustment reading operation prior to normal reading operation of asubject image. The main controller 153 controls the sensitivity settingregister 159 to set an image reading sensitivity for sensitivityadjustment reading operation via the data controller 152. The maincontroller 153 reads a subject image for the purpose of sensitivityadjustment.

In general, this sensitivity adjustment reading operation is performedimmediately before normal image reading operation. However, sensitivityadjustment reading operation is not necessarily executed at a timingprior to normal image reading operation, and may be independentlyperformed at a different timing from normal image reading operation, forexample, when an image reading sensitivity must be set.

Similar to normal image reading operation, sensitivity adjustmentreading operation is achieved by executing a series of processes: resetoperation→charge accumulating operation→pre-charge operation→readingoperation for double-gate photosensors which constitute the photosensorarray. At this time, the charge accumulating period is changed stepwiseby repetitively executing pre-charge operation and reading operation atpredetermined timings for respective rows such that a higher imagereading sensitivity is set for a larger row number in the photosensorarray 100 in which double-gate photosensors are arrayed in a matrix of256 rows×196 columns, as shown in FIG. 4. Image data read at differentimage reading sensitivities for respective rows are acquired by onereading operation of a subject image. An image reading sensitivity foreach row is stored in the form of a table (row number vs. image readingsensitivity correspondence table) in the RAM 160 in correspondence withthe row number. A longer charge accumulating period and higher imagereading sensitivity are set for a larger row number. A ridge/valleypattern of a fingerprint is read as a faint (light) image or an almostinvisibly bright image under the influence of external light (upward inFIG. 4). On the other hand, a shorter charge accumulating period andlower image reading sensitivity are set for a smaller row number. Theridge/valley pattern of the fingerprint is read as a blackish image oran almost invisibly dark image (downward in FIG. 4). Note that adetailed method of setting an image reading sensitivity in sensitivityadjustment reading operation will be described later.

In this embodiment, the image reading sensitivity is changed every rowof the photosensor array in sensitivity adjustment reading operation.The present invention is not limited to this, and the image readingsensitivity may be changed stepwise, e.g., every plurality of rows.Alternatively, a subject image may be read by one frame of thephotosensor array every time the image reading sensitivity is changed.In short, image data suffices to be obtained at different image readingsensitivities for one subject.

(Step S12)

Image data read by the above-described sensitivity adjustment readingoperation is converted into a digital signal via the amplifier 133 ofthe drain driver 130 and the A/D converter 140. The digital signal isinput to the data comparator 154 as pixel data (lightness data) of eachimage reading sensitivity that corresponds to the bright/dark pattern ofthe subject image.

More specifically, as shown in FIGS. 5A to 5E, 256 gray levels are setbetween white and black on the subject image. Voltage changes in drainvoltage VD in, e.g., the 176th, 152nd, 128th, 104th, and 80th rows inthe photosensor array 100 as shown in FIG. 4 are converted intolightness data values ranging from 0 to 255, which are graphed. In the176th row, as shown in FIG. 5A, the sensitivity is set high, so thatlightness data substantially converges to the upper limit (255) andhardly provides any information as image data. In the 152nd row, asshown in FIG. 5B, the sensitivity is set relatively high, lightness datareaches the upper limit on some columns, and all the ridge/valley(bright/dark) patterns of image data cannot be read.

To the contrary, in the 128th row, as shown in FIG. 5C, lightness datadoes not reach either the upper limit (255) or lower limit (0) on allthe columns, and is distributed between the upper and lower limits. Inthe 104th row, as shown in FIG. 5D, the sensitivity is set relativelylow, and lightness data is distributed between the upper and lowerlimits. However, lightness data reaches the lower limit on some columns,and all the ridge/valley patterns of image data cannot be read. In the80th row, as shown in FIG. 5E, the sensitivity is set low, so thatlightness data substantially converges to the lower limit and hardlyprovides any information as image data.

(Steps S13 & S14)

Lightness data (pixel of highest gray level) representing a maximumvalue and lightness data (pixel of lowest gray level) representing aminimum value are extracted from the lightness data input to the datacomparator 154 for each image reading sensitivity, and output to theadder 155.

As shown in FIG. 6A, lightness data (pixel of highest gray level)representing a maximum value and lightness data (pixel of lowest graylevel) representing a minimum value are extracted for each row from thelightness data input to the data comparator 154, and output to the adder155.

The adder 155 calculates the difference between the maximum and minimumvalues of lightness data for each row, thus obtaining a dynamic rangefor each image reading sensitivity. The adder 155 stores the dynamicrange in the RAM 156 via the data selector 160. The adder 155 executesdynamic range calculation processing for all the rows or a predeterminednumber of rows.

(Step S15)

Dynamic ranges for respective image reading sensitivities stored in theRAM 160 are read out via the data selector 156, and input to the datacomparator 154. The data comparator 154 extracts a maximum value fromdynamic ranges for respective image reading sensitivities.

More specifically, maximum and minimum values are extracted as numericaldata on the basis of distribution changes in lightness data ofrespective rows shown in FIGS. 5A to 5E. Dynamic ranges are calculatedfrom the differences between the maximum and minimum values. As shown inFIG. 6A, the data range depends on the minimum value in the 176th and152nd rows because lightness data reaches the upper limit and itsmaximum value is fixed to 255. In the 104th and 80th rows, the datarange depends on the maximum value because lightness data reaches thelower limit and its minimum value is fixed to 0.

In the 128th row, the dynamic range depends on the difference betweenthe maximum and minimum values of lightness data because the lightnessdata does not reach either the upper or lower limit. The 128th rowprovides a larger data range than those of the 176th, 152nd, 104th, and80th rows. In other words, lightness data of the 128th row is image datawith a good contrast corresponding to the ridge/valley pattern of afingerprint, and an optimal image reading sensitivity can be determinedto be set.

(Step S16)

An image reading sensitivity corresponding to the maximum dynamic rangeis extracted and set as a standard image reading sensitivity.

More specifically, as shown in FIG. 6B, an image reading sensitivity setfor the 128th row, i.e., the charge accumulating period T₁₂₈ of thedouble-gate photosensor is extracted by looking up a row number vs.image reading sensitivity correspondence table stored in the RAM 160 onthe basis of a row number (128th row) at which the dynamic rangemaximizes.

(Step S17)

The main controller 153 controls the data controller 152 so as toexecute processing of correcting the image reading sensitivity (standardimage reading sensitivity) on the basis of a predeterminedperiod-increase rate set in advance in the period-increase register 157.

For example, the image reading sensitivity which is set for the 128throw and exhibits a maximum dynamic range corresponds to the chargeaccumulating period T₁₂₈=91.2 ms, and the period-increase rate set inadvance in the period-increase register 157 is 32%. In this case, acharge accumulating period used for rewrite of the sensitivity settingregister 159 is a 32%-period-increased numerical value:91.2×(1+0.32)≈120 ms   (2)

This period-increase correction processing increases by 1.32 times theimage reading sensitivity of the row which is extracted in step S16 andexhibits the maximum dynamic range. The charge accumulating period isprolonged by 1.32 times so as to increase the amount of excitation light(visible light) incident on each double-gate photosensor. Theperiod-increase rate of about 32% is a numerical value which has beenfound as a result of various experiments by the present inventors to beable to correct the image reading sensitivity to an optimal one whichcan relatively accurately read a fingerprint even if the subject is akeratinized finger (to be described later) in the sensitivity adjustmentapparatus and method according to the first embodiment.

(Step S18)

The data controller 152 controls rewrite of the sensitivity settingregister 159 to set the corrected image reading sensitivity (chargeaccumulating period). Sensitivity adjustment processing based onsensitivity adjustment reading operation ends.

After that, normal image reading operation of a subject image isexecuted based on the optimal image reading sensitivity (chargeaccumulating period) determined by the above-described sensitivityadjustment reading operation and image reading sensitivity adjustmentoperation.

The read image of a fingerprint and the image reading sensitivity whenthe sensitivity adjustment apparatus and method of the photosensorsystem are applied to a fingerprint reading apparatus will beexemplified with reference to the several views of the accompanyingdrawing. A case wherein no period-increase processing is executed willalso be explained for comparison in order to show the effectiveness ofperiod-increase correction processing in the sensitivity adjustmentmethod according to the first embodiment.

FIGS. 7A and 7B are a view showing the read image of a fingerprint, anda graph showing lightness data of respective columns at an image readingsensitivity when normal image reading operation is performed at thestandard image reading sensitivity without executing period-increasecorrection processing of the first embodiment in the sensitivityadjustment method of the photosensor system. FIGS. 8A and 8B are a viewshowing the read image of a fingerprint, and a graph showing lightnessdata of respective columns at an image reading sensitivity when thestandard image reading sensitivity undergoes period-increase correctionprocessing of the first embodiment in the sensitivity adjustment methodof the photosensor system and normal image reading operation isperformed at the resultant image reading sensitivity. FIGS. 7A and 8Ashow rough subject images when reading operation is executed based oneach predetermined image reading sensitivity. The ridge portion(projecting region) of a fingerprint is illustrated in white (orwhitish), and the valley line (recessed region) of the fingerprint isillustrated in black (or blackish).

A case wherein no period-increase processing is done in the abovesensitivity adjustment method will be explained.

As described in “Description of the Related Art”, an appropriate imagereading sensitivity may not be set in accordance with the state of theskin surface layer of a finger serving as a subject in an application ofthe photosensor system to a finger reading apparatus. The skin surfacelayer of a finger often keratinizes, the amount of light reflected bythe ridge portion of the fingerprint increases, and the brightness isobserved locally high. Compared to a case wherein the fingerprint of afinger with a non-keratinized skin is read, a relatively large dynamicrange can be obtained at a relatively low image reading sensitivitywithin a short charge accumulating period. An image reading sensitivity(standard image reading sensitivity) corresponding to a maximum dynamicrange is extracted as an optimal image reading sensitivity on the basisof the read fingerprint image data. In this case, a lower image readingsensitivity than an originally optimal image reading sensitivity, i.e.,a shorter charge accumulating period than an originally optimal chargeaccumulating period is extracted and set. If normal fingerprint readingoperation is executed based on this image reading sensitivity, a subjectimage is read as a relatively dark image and the ridge portion in theimage tends to be discontinuous, as shown in FIG. 7A, because the chargeaccumulating period is set shorter than an originally optimal chargeaccumulating period. The distribution characteristic of lightness dataof an arbitrary row (e.g., 81st row) was examined for the subject image,and found to exhibit a lightness data distribution with a large dynamicrange, as shown in FIG. 7B. This is because the presence of pixels(columns) having extremely high maximum values of lightness data underthe influence of the ridge portion of a keratinized skin surface layer.This distribution does not accurately correspond to the original imagepattern of the fingerprint. Hence, a fingerprint cannot be accuratelyread, decreasing the precision of fingerprint collation processing inthe external function section.

To the contrary, the sensitivity adjustment apparatus and method of thephotosensor system according to the first embodiment execute correctionprocessing based on a predetermined period-increase rate (e.g., about30%) for an image reading sensitivity (standard image readingsensitivity) which provides a maximum dynamic range and is obtained bysensitivity adjustment reading operation and image reading sensitivityadjustment operation for fingerprint image data of a finger whose skinsurface layer is keratinized. A higher image reading sensitivity, i.e.,a longer charge accumulating period is set in comparison with the imageshown in FIG. 7A. Accordingly, an image reading sensitivity closer to anoriginally optimal image reading sensitivity is set.

By executing normal fingerprint reading operation on the basis of thecorrected image reading sensitivity, a subject image is read as an imagehaving a proper brightness, and a high-quality image having almost nodiscontinuous ridge portion can be attained, as shown in FIG. 8A. Thedistribution characteristic of lightness data of an arbitrary row (e.g.,81st row) was examined for the subject image, and found to exhibit alightness data distribution with a relatively large dynamic range, asshown in FIG. 8B. In addition, extreme lightness data under theinfluence of a keratinized skin surface layer was observed at neithermaximum nor minimum values. A stable image accurately corresponding tothe original image pattern of the fingerprint was obtained. As a result,fingerprint collation processing can be executed at high precision inthe external function section 200.

According to the sensitivity adjustment apparatus and method of thefirst embodiment, sensitivity adjustment reading operation is performedusing an actual subject prior to normal image reading operation, whilethe image reading sensitivity is changed stepwise for each row. A row inan optimal image reading state is easily determined based on the dynamicrange of lightness data of each row, and an image reading sensitivity(standard image reading sensitivity) set for the determined row isextracted. Then, correction processing based on a predeterminedperiod-increase rate is executed to set an optimal image readingsensitivity. Even when the brightness of a subject changes depending onchanges in environmental illuminance, or a fluctuation factor or trendexists due to the individual difference of a subject (individualdifference such as gender or age) or external environment (temperature,humidity, or the like), sensitivity adjustment processing is uniquelyexecuted by a simple method to greatly suppress the influence of theenvironmental illuminance of external light or keratinization of a skinsurface layer. An optimal image reading sensitivity can be set, and asubject image can be accurately read by normal image reading operation.

In this embodiment, sensitivity adjustment processing has been describedfor only the 176th, 152nd, 128th, 104th, and 80th rows shown in FIGS. 5Ato 5E as representative rows. Sensitivity adjustment processing may beexecuted for all the 256 rows which constitute the photosensor array 100or for specific rows (e.g., every 24 rows in FIGS. 5A to 5E).

When sensitivity adjustment processing is executed for all the rows, amore optimal image reading sensitivity can be extracted from imagereading sensitivities set for respective rows. If sensitivity adjustmentprocessing is executed for only specific rows, the number of datasubjected to a series of processing operations can be greatly decreasedto reduce the processing burden on the controller 150 and shorten theprocessing time. Sensitivity adjustment reading operation can quicklyshift to normal reading operation of a subject image.

In this embodiment, a subject image (fingerprint image) is read by theentire sensing surface of the photosensor array 100 in sensitivityadjustment reading operation. The present invention is not limited tothis, and sensitivity adjustment processing may be done by reading asubject image only in a detection region 40 formed from a predeterminedrestricted region, as shown in FIGS. 15A and 15B in the third embodiment(to be described later). In this case, the reading time can beshortened, and the number of data subjected to processing operation canbe further decreased to shorten the processing time. Sensitivityadjustment reading operation can more quickly shift to normal readingoperation of a subject image.

In the above-mentioned embodiment, of lightness data of respective rowsacquired by sensitivity adjustment reading operation, an image readingsensitivity set for a row having a maximum dynamic range uniquelyundergoes period-increase correction processing on the basis of apredetermined period-increase rate set in advance in the period-increaseregister 157. However, the present invention is not limited to this.

For example, the period-increase rate set in the period-increaseregister 157 may be arbitrarily set by input operation outside thephotosensor system (or controller 150). This is achieved by operatingthe input/output interface device (not shown) of the external functionsection 200, inputting an arbitrary period-increase rate as numericaldata, and rewriting the period-increase register 157.

As an example to which the arrangement of arbitrarily rewriting theperiod-increase value is suitable applied, the photosensor system isapplied to a fingerprint reading apparatus. In this arrangement, whenthe type of subject is restricted or specified to a certain degree, andthe age or gender of an individual serving as a subject or an externalenvironment such as a temperature or humidity is grasped in advance, aproper period-increase rate is set in accordance with such a fluctuationfactor or trend every time or by inputting a numerical value in advance.A more optimal image reading sensitivity can be set while the influenceof the individual difference or external environment is suppressed. Asubject image can be accurately read.

Correction based on the period-increase rate is always executed in thefirst embodiment, but the present invention is not limited to this. Forexample, in an arrangement in which the photosensor system is applied toa fingerprint reading apparatus, an optimal image reading sensitivity isextracted and set without applying correction processing based on theperiod-increase rate. Then, normal subject image reading operation isdone to perform fingerprint collation processing. If the result isrejected, correction processing based on the period-increase rate isapplied to correct the image reading sensitivity. At the corrected imagereading sensitivity, normal subject image reading operation is executedagain to perform fingerprint collation processing. In this case, normalsubject image reading operation can be quickly achieved without anyprocessing time of correction processing unless the skin state of asubject (fingerprint) is keratinized.

Second Embodiment

The second embodiment of a photosensor system according to the presentinvention will be described with reference to the several views of theaccompanying drawing.

FIG. 9 is a block diagram showing an arrangement of a controller appliedto the photosensor system according to the second embodiment. The samereference numerals as in the first embodiment shown in FIG. 2 denote thesame parts, and a description thereof will be simplified or omitted.

Similar to the sensitivity adjustment method according to the firstembodiment, the second embodiment extracts the image reading sensitivityof a row having a maximum dynamic range in a lightness data distributionobtained by sensitivity adjustment reading operation. Then, whether toexecute period-increase correction processing for the extracted imagereading sensitivity is determined, and a more optimal image readingsensitivity is set.

As shown in FIG. 9, a controller 150 in the second embodiment comprisesa device controller 151, data controller 152, main controller 153, datacomparator 154, adder 155, data selector 156, period-increase register157, and sensitivity setting register 159, all of which have the samearrangements as those in the first embodiment shown in FIG. 2. Inaddition, the controller 150 comprises a mean compare value register 158which holds a mean compare value serving as a reference value fordetermining whether to execute period-increase correction of the maincontroller 153 for an image reading sensitivity which is extracted bythe data comparator 154, adder 155, and data selector 156 andcorresponds to a maximum dynamic range. In the second embodiment, themean compare value held in the mean compare value register 158 isuniquely set based on a maximum dynamic range out of a lightness datadistribution obtained by sensitivity adjustment reading operation of asubject image.

Processing operation by the controller will be explained with referenceto the several views of the accompanying drawings.

FIG. 10 is a flow chart showing an example of sensitivityadjustment/setting processing executed by the controller applied to thephotosensor system according to the second embodiment. A description ofthe same processing steps as those in the first embodiment shown in FIG.3 will be simplified or omitted.

(Steps S21 to S26)

As shown in FIG. 10, similar to steps S11 to S16 in the firstembodiment, different image reading sensitivities are set for respectiverows at a timing prior to normal reading operation of a subject.Sensitivity adjustment subject image reading operation is executed toobtain pixel data (lightness data) of each image reading sensitivitythat corresponds to the bright/dark pattern of the subject image.Lightness data representing maximum and minimum values are extractedfrom the lightness data for each image reading sensitivity (e.g., foreach row). The difference between the maximum and minimum values iscalculated to attain a dynamic range for each image reading sensitivity,and the dynamic range is stored in a RAM 160. A maximum value isextracted from dynamic ranges for respective image reading sensitivitiesthat are stored in the RAM 160, and a corresponding image readingsensitivity (charge accumulating period) is extracted.

(Steps S27 & S28)

The main controller 153 controls the data controller 152 so as tocalculate the mean value of pixel data (lightness data) corresponding tothe image reading sensitivity at which the dynamic range maximizes,i.e., the image reading sensitivity extracted in step S26, and tocalculate the median value (mean compare value) of the dynamic range.

The calculated mean value of lightness data is temporarily stored in theRAM 160 via the data selector 156. The median value of the dynamic rangeis held in the mean compare value register 158 via the data selector 156and data controller 152.

(Step S29)

The main controller 153 controls the data controller 152 so as toexecute processing of comparing the mean value of lightness data whichis calculated in step S27 and stored in the RAM 160 with the medianvalue of the maximum dynamic range which is calculated in step S28 andheld in the mean compare value register 158.

If a keratinized finger is read as a subject, the amount of lightreflected by the ridge portion of the keratinized finger may increase,and the brightness may be observed locally high, as shown in FIG. 7B. Inthis case, lightness data appears locally high. For this reason,lightness data of a row determined to have a maximum dynamic range underthe influence of keratinization may be merely a set of small lightnessdata except locally high data. From this, when the mean value oflightness data is relatively small, even a row having a maximum dynamicrange is determined to be a set of small lightness data under theinfluence of keratinization. An image reading sensitivity correspondingto this row can be determined to be lower than an originally optimalimage reading sensitivity.

Whether the mean value of lightness data in a row exhibiting a maximumdynamic range is small or large is determined by comparing the meanvalue of lightness data with the median value of the dynamic range orcomparing the sum of lightness data of this row with the product of themedian value of the dynamic range and the number of columns.

If the mean value of lightness data (or the sum of lightness data) issmaller than the median value of the dynamic range (or the product ofthe median value and the number of columns), the row can be determinedto have an improper lightness data distribution under the influence ofkeratinization. If the means value is larger, the row can be determinedto have a proper lightness data distribution free from any influence ofkeratinization. The following processing operation is executed based onthis determination result.

(Steps S30 & S31)

If the mean value of lightness data at an image reading sensitivityhaving a maximum dynamic range is determined in step S29 to be smallerthan the median value of the dynamic range, the main controller 153determines that the lightness data distribution at this image readingsensitivity is improper. The main controller 153 controls the datacontroller 152 so as to execute period-increase correction processingfor the image reading sensitivity having the dynamic range on the basisof a predetermined period-increase rate set in advance in theperiod-increase register 157 or arbitrarily input and set via anexternal function section 200 or the like.

A value corrected by increasing the image reading sensitivity, which isextracted in step S26 and has the maximum dynamic range, by theperiod-increase rate in period-increase correction processing is set asan image reading sensitivity. That is, the charge accumulating period isprolonged by a time corresponding to the period-increase rate.

Then, the data controller 152 rewrites the sensitivity setting register159 to set the corrected image reading sensitivity (charge accumulatingperiod).

(Step S32)

If the mean value of lightness data at an image reading sensitivityhaving a maximum dynamic range is determined in step S29 to be largerthan the median value of the dynamic range, the main controller 153determines that the lightness data distribution at this image readingsensitivity is proper. The data controller 152 rewrites the sensitivitysetting register 159 without performing period-increase correction, andsets the image reading sensitivity (charge accumulating period)extracted in step S26.

The sensitivity adjustment apparatus and method of the photosensorsystem can determine whether to execute period-increase correctionprocessing for an extracted image reading sensitivity in accordance withwhether lightness data at an image reading sensitivity having a maximumdynamic range is improper under the influence of keratinization of afinger or the like (fluctuation factor or trend such as the individualdifference of a subject or an external environment) in image readingsensitivity adjustment operation of setting an optimal image readingsensitivity (charge accumulating period) on the basis of the dynamicrange of lightness data for each image reading sensitivity obtained bysensitivity adjustment reading operation executed prior to normal imagereading operation. A proper image reading sensitivity can be set byexecuting correction processing only for an improper image readingsensitivity.

In the second embodiment, when whether to execute period-increasecorrection based on a predetermined period-increase rate is determinedfor an image reading sensitivity having a maximum dynamic range out oflightness data for respective image reading sensitivities acquired bysensitivity adjustment reading operation, the median value of thedynamic range is uniquely set for comparison with the mean value oflightness data at the image reading sensitivity. However, the presentinvention is not limited to this.

For example, the median value of the dynamic range may be used as adefault value (reference value), and the mean compare value set in themean compare value register 158 may be arbitrarily changed and set byinput operation outside the photosensor (or controller 150).

Similar to the period-increase register 157 described in the firstembodiment, the mean compare value register 158 is rewritten byoperating the input/output interface device (not shown) of the externalfunction section 200, inputting an arbitrary mean compare value asnumerical data, and rewriting the mean compare value register 158. As anexample to which the arrangement of arbitrarily rewriting the meancompare value is suitable applied, the photosensor system is applied toa fingerprint reading apparatus. In this arrangement, when fingerprintcollation processing is executed in the external function section 200 byusing as a reference the ridge portion (e.g., portions observed in whitein FIGS. 7A and 8A) of the image pattern of a fingerprint, the meancompare value is set to a relatively large value so as to facilitateexecution of the above-described period-increase correction processing.

As another example, if the humidity is low in an arrangement whichallows grasping an external environment such as a temperature in advanceby a sensor or the like, the mean compare value is set to a relativelylarge numerical value because the skin surface layer readilykeratinizes. This facilitates execution of period-increase correctionprocessing described above.

As still another example, the mean compare value is set to a relativelylarge numerical value for an individual exhibiting a low authenticationprobability of fingerprint collation processing in an arrangement whichenables grasping the feature of an individual serving as a subject by anID card or the like. This also facilitates execution of theabove-mentioned period-increase correction processing.

Accordingly, a more optimal image reading sensitivity can be set toaccurately read a subject image while the influence of an individualdifference or external environment is suppressed.

Third Embodiment

The third embodiment of a photosensor system according to the presentinvention will be described with reference to the several views of theaccompanying drawing.

FIG. 11 is a block diagram showing the whole arrangement of thephotosensor system according to the third embodiment. A double-gatephotosensor shown in FIG. 26A is adopted as a photosensor, and thearrangement of the photosensor system shown in FIG. 27 will be referredto, as needed. The same reference numerals as in the photosensor systemshown in FIG. 1 denote the same parts, and a description thereof will besimplified or omitted.

As shown in FIG. 11, similar to the photosensor system shown in FIG. 1,the photosensor system according to the third embodiment roughlycomprises a photosensor array 100, a top gate driver 110, a bottom gatedriver 120, an output circuit section 130 which is made up of a columnswitch 131, pre-charge switch 132, and amplifier 133, an A/D converter140, a controller 150, and a RAM 160 for temporarily storing acquiredimage data (pixel data group), and processing data or the like relatingto sensitivity setting processing. The photosensor system furthercomprises a ROM 170 which holds the control program of the controller150 and various control data.

Similar to the first and second embodiments, the controller 150 has afunction of performing sensitivity adjustment reading operation for asubject, and controlling extraction and setting of an image readingsensitivity capable of accurately reading a subject image on the basisof the read image data. In addition, as will be described below, thecontroller 150 has a function of removing specific pixel data from imagedata (pixel data group) of a subject image read for sensitivityadjustment in sensitivity adjustment reading operation in order toprevent or suppress the influence of abnormal pixel data caused by aforeign matter such as dust deposited on the sensing surface of thephotosensor array or the influence of an element defect present on aphotosensor array.

The detailed arrangement and operation of the controller applied to thephotosensor system according to the third embodiment will be explainedin more detail with reference to the several views of the accompanyingdrawing.

FIG. 12 is a block diagram showing an arrangement of the controllerapplied to the photosensor system according to this embodiment.

As shown in FIG. 12, similar to the first embodiment shown in FIG. 2,the controller 150 in the third embodiment comprises a device controller151 for controlling the operations of the top gate driver 110, bottomgate driver 120, and pre-charge switch 132, a data controller 152 forperforming write/readout of data in/from the RAM 160 and ROM 170, andmanaging various data, and a main controller 153 which supervises thecontrollers 151 and 152 in accordance with a predetermined controlprogram, interfaces with an external function section 200, and exchangescontrol signals. The controller 150 further comprises: a data comparator154 for extracting maximum and minimum values by comparing the sizes ofinitial pixel data (to be also referred to as “original pixel data”hereinafter for descriptive convenience) contained in image data inputas a digital signal from the photosensor array 100 via the A/D converter140, or the sizes of pixel data (to be also referred to as “processedpixel data” hereinafter for descriptive convenience) obtained afterspecific pixel data undergoes removal processing by specific pixel dataremoval operation (to be described later), and for extracting a maximumdynamic range calculated by an adder 155; the adder 155 for calculatinga dynamic range from the difference between the maximum and minimumvalues of processed pixel data extracted by the data comparator 154; adata selector 156 for receiving image data and processing data processedvia the A/D converter 140, data comparator 154, and adder 155, andswitching between write/readout of these data in/from the RAM 160 asneeded, re-input of these data to the data comparator 154 and adder 155,and output of these data to the external function section 200 via thedata controller 152; and a sensitivity setting register 159 for settingthe timings of control signals φtg and φbg to be output from the devicecontroller 151 to the top and bottom gate drivers 110 and 120 so as tooptimize the image reading sensitivity of the photosensor array 100 onthe basis of a control signal from the data controller 152.

The main controller 153 has the following function of removing specificpixel data. More specifically, the main controller 153 writes, in apredetermined storage area of the RAM 160 via the data selector 156,only a pixel data group except specific pixel data which is based on apredetermined condition and extracted by the data comparator 154, e.g.,pixel data having a maximum or minimum value out of original pixel data.As a result, the main controller 153 can generate a pixel data(processed pixel data) group from which the pixel data having a maximumor minimum value is removed. Specific pixel data removal operation ofthe main controller 153 will be described in detail later.

A method of setting an optimal image reading sensitivity by thesensitivity setting register 159 in the photosensor array 100, top gatedriver 110, and bottom gate driver 120 will be described in detail. Thismethod also comprises the same steps as those in the first and secondembodiments. That is, a dynamic range is calculated for each imagereading sensitivity by comparing the sizes of lightness data containedin processed pixel data for each image reading sensitivity generatedfrom image data (original pixel data) input from the photosensor array.An image reading sensitivity which yields a maximum dynamic range is setas an optimal value.

Processing operation by the controller having the above-mentionedarrangement will be explained with reference to the several views of theaccompanying drawings.

FIG. 13 is a flow chart showing an example of sensitivityadjustment/setting processing executed by the controller applied to thephotosensor system according to the third embodiment. A description ofthe same processing steps as those in the first embodiment shown in FIG.3 will be simplified or omitted.

A series of following processing steps are implemented by loading acontrol program stored in advance in the ROM 170 to the RAM 160 andexecuting the program by the controller 150.

(Step S41)

As shown in FIG. 13, the main controller 153 starts sensitivityadjustment reading operation at a timing prior to normal readingoperation of a subject image. The main controller 153 sets an imagereading sensitivity for sensitivity adjustment reading operation in thesensitivity setting register 159 via the data controller 152. The maincontroller 153 reads a sensitivity adjustment subject image placed on asensing surface defined on one surface of the photosensor array 100.This sensitivity adjustment reading operation is performed while thecharge accumulating period is changed stepwise every row of thephotosensor array 100 so as to increase the image reading sensitivityfor a larger row number, as shown in FIG. 4. Hence, image data read atdifferent image reading sensitivities are acquired by one readingoperation.

(Step S42)

Image data read by sensitivity adjustment reading operation is convertedinto a digital signal via the A/D converter 140, and the digital signalis temporarily stored in the RAM 160 via the data selector 156.Thereafter, the data controller 152 inputs the digital data as originalpixel data (lightness data) of each image reading sensitivity to thedata comparator 154 via the data selector 156.

(Step S43)

The data comparator 154 compares the sizes of lightness data of inputoriginal pixel data for each image reading sensitivity. The datacomparator 154 extracts specific pixel data which satisfies apredetermined condition, i.e., pixel data having a maximum lightnessdata value or a plurality of pixel data in descending order of lightnessdata values.

(Step S44)

Pixel data having a maximum value extracted in step S43 or apredetermined number of pixel data containing the maximum value areremoved from an original pixel data group for each image readingsensitivity. The remaining image data group is stored in a predeterminedstorage area of the RAM 160, generating a pixel data group (processedpixel data) obtained by removing the specific pixel data from theoriginal pixel data group. A detailed example of specific pixel dataremoval operation will be described later.

(Steps S45 & S46)

The data controller 152 extracts the processed pixel data for each imagereading sensitivity, and loads the data into the data comparator 154 viathe data selector 156. The data comparator 154 compares the sizes of theprocessed pixel data for each image reading sensitivity, and extractspixel data having maximum and minimum lightness data values.

(Step S47)

The adder 155 controlled by the data controller 152 calculates thedifference between the maximum and minimum lightness data values ofpixel data extracted for each image reading sensitivity, therebyobtaining a dynamic range for each image reading sensitivity. The resultis temporarily stored in the RAM 160 via the data selector 156.

This dynamic range calculation processing is executed for respectiveimage reading sensitivities.

(Step S48)

Dynamic ranges for respective image reading sensitivities stored in theRAM 160 are read out via the data selector 156, and loaded into the datacomparator 154. The data comparator 154 compares the dynamic ranges forrespective image reading sensitivities, and extracts a maximum dynamicrange.

(Step S49)

The sensitivity setting register 159 is rewritten via the datacontroller 152 to set an image reading sensitivity corresponding to theextracted maximum dynamic range as an optimal image reading sensitivity.

Sensitivity adjustment reading operation (step S41) applied to theabove-described processing operation of the controller will be explainedin detail with reference to the several views of the accompanyingdrawing.

FIGS. 14A and 14B are conceptual views showing a target region and anexample of reading operation in sensitivity adjustment reading operationaccording to the third embodiment. FIGS. 15A and 15B are conceptualviews showing another target region and another example of readingoperation in sensitivity adjustment reading operation according to thethird embodiment.

Sensitivity adjustment reading operation applied to the third embodimentis executed as follows. On the basis of control signals and an imagereading sensitivity which are output from the device controller 151 andset in the top gate driver 110, bottom gate driver 120, and pre-chargeswitch 132, a sensitivity adjustment image placed on the sensing surfaceis read within an effective reading region 30 formed from the entiresensing surface of the photosensor array 100, as shown in FIGS. 14A and14B, or within a detection region 40 formed from a predetermined regionrestricted and set in advance within the effective reading region 30, asshown in FIGS. 15A and 15B.

Image reading sensitivities set in double-gate photosensors 10 whichconstitute the photosensor array 100 may be set to differ from eachother, e.g., stepwise every row, every predetermined number of rows, orevery reading region such as every effective reading region 30 or everydetection region 40.

As a detailed driving method of the photosensor 10 in sensitivityadjustment reading operation, all the rows in the photosensor array 100made up of 256×196 pixels may be sequentially read, as shown in FIG.14A. Alternatively, every plurality of rows (in this case, every 10rows) such as the 10th, 20th, . . . , 180th, and 190th rows may besequentially read, as shown in FIG. 14B.

For example, all the rows in the detection region 40 formed from a rowrange of 64th to 191st rows and a column range of 67th to 130th columnsmay be sequentially read, as shown in FIG. 15A. Alternatively, everyplurality of rows (in this case, every 10 rows) such as 70th, 80th, . .. , 180th, and 190th rows may be sequentially read, as shown in FIG.15B.

In the third embodiment, a region to be read in normal image readingoperation of a subject may be directly set as a region (effectivereading region 30 or detection region 40) subjected to sensitivityadjustment reading operation. As the detection region 40, the number ofpixels included in the region (row and column ranges) may be so set asto have at least a minimum number of pixel data necessary tosatisfactorily execute processing operation in specific pixel dataremoval operation and optimal image reading sensitivity settingoperation which are applied to this embodiment. A region subjected tosensitivity adjustment reading operation according to the thirdembodiment is not limited to the setting region (128 rows from 64th to191st rows and columns from 67th to 130th columns) of the detectionregion 40. A region defined by narrower (smaller number of pixels) rowand column ranges may be set. Setting of a minimum region which can beset as a detection region will be verified later.

Specific pixel data removal operation (steps S43 and S44) applied toprocessing operation of the controller will be exemplified.

Of specific pixel data removal operation applied to this embodiment,operation of extracting one or a plurality of pixel data in descendingorder of lightness data values can adopt the following method.

More specifically, in step S43 described above, the lightness datavalues of original pixel data loaded into the data comparator 154 arecompared for each image reading sensitivity. The original pixel data aresorted in descending order of lightness data values and stored in theRAM 160 via the data selector 156.

In steps S44 to S46, when pixel data stored in the RAM 160 to calculatea dynamic range are to be loaded into the data comparator 154 again, apredetermined number of pixel data from the first to n-th (n is anarbitrary natural number) pixel data are uniquely designated and removedin descending order of lightness data values from an original pixel datagroup sorted in the above-described way every image reading sensitivity.The remaining pixel data group (corresponding to processed pixel data)is loaded into the data comparator 154. Of the loaded pixel data group,pixel data having the largest lightness data value, i.e., pixel datasecond largest to the designated/removed pixel data, and pixel datahaving the smallest lightness data value are extracted and read out viathe data selector 156. The adder 155 calculates the difference betweenthe maximum and minimum values of the pixel data, i.e., a dynamic range.

By this simple processing of comparing pixel data and sorting them bytheir sizes, pixel data having a maximum lightness data value (highestlightness) or a predetermined number of pixel data in descending orderof lightness data values are uniquely removed from an original pixeldata group. In addition, pixel data having maximum and minimum valuesare extracted from a processed pixel data group.

A case wherein different image reading sensitivities are set forrespective rows will be explained.

In step S103, the sizes of pixel data of pixels adjacent to each otheron the photosensor array out of an original pixel data group loaded intothe data comparator 154 every row, i.e., every image reading sensitivityare so compared as to sequentially extract, e.g., pixel data from thefirst pixel of the target row in descending size order.

More specifically, as shown in (a) and (b) of FIG. 16, pixel data of thefirst pixel (first pixel data) and pixel data of the second pixel(second pixel data) are compared in the pixel data group (m pixel data)of one row to extract pixel data having a larger lightness data value(higher lightness). The extracted pixel data and pixel data of the thirdpixel (third pixel data) are compared to extract pixel data having alarger lightness data value. This processing is executed up to the m-thpixel data of the target row.

By comparing pixel data of adjacent pixels, only pixel data having alarger lightness data value is always extracted and subjected to thenext comparison processing. This comparison/extraction processing isrepetitively executed for the pixel data group of one row, extractingonly pixel data having a maximum lightness data value (highestlightness).

Then, as shown in (c) and (d) of FIG. 16, pixel data having a smallerlightness data value (lower lightness) is extracted incomparison/extraction processing described above. Extracted pixel dataare sequentially stored in a predetermined storage area of the RAM 160via the data selector 156. After pixel data having a maximum lightnessdata value is extracted, the pixel data group is loaded into the datacomparator 154 again. Similar to the above-describedcomparison/extraction processing, the first pixel data (pixel datahaving a lower lightness as a result of comparing the first and secondpixel data) is compared with the second pixel data (pixel data having alower lightness as a result of comparing the third pixel data with pixeldata having a higher lightness out of the first and second pixel data),extracting pixel data having a larger lightness data value (higherlightness). The extracted pixel data is compared with the third pixeldata, extracting pixel data having a larger lightness data value. Thisprocessing is executed up to the (m-1)th pixel data of the target row.

In this manner, processing of always extracting only pixel data having alarger lightness data value as a result of comparing pixel data in aremaining pixel data group from which pixel data having a maximumlightness data value (highest lightness) is removed is repetitivelyexecuted. Only pixel data having the second largest lightness data valueis extracted for the target image reading sensitivity (row). This pixeldata comparison/extraction processing is repetitively executed i times(i is an arbitrary natural number). A predetermined number of pixel dataup to the i-th pixel data are extracted in descending order of lightnessdata values.

Setting of removing only one pixel data having a maximum lightness datavalue from an original pixel data group for each image readingsensitivity is effective for eliminating the influence of abnormal pixeldata caused by the element defect of a photosensor which constitutes aphotosensor array. The number of element defects occurred at actualproduct level in a photosensor system applicable to the third embodimentis at most one per row which constitutes a photosensor array. Since aplurality of element defects hardly occur in one row, the presentinvention can satisfactorily employ a simple method of removing onlypixel data having a maximum lightness data value.

A method of removing a predetermined number of (plurality of) pixel datain descending order of lightness data values from an original pixel datagroup for each image reading sensitivity can be effectively applied toeliminating the influence of a relatively large foreign matter depositedon the sensing surface on the photosensor array or the influence ofabnormal pixel data caused by an element defect generated over aplurality of pixels.

Each method applied to the above-mentioned specific pixel data removaloperation has been described by exemplifying only a method of uniquelyextracting and removing, from an original pixel data group, pixel datahaving a maximum lightness data value or a predetermined number of pixeldata having large lightness data values containing a maximum value inthe presence of an abnormal pixel serving as a bright spot (bright spotdefect) owing to the element defect of a photosensor which constitutes aphotosensor array, variations in element characteristics, or depositionof a foreign matter on the sensing surface. The present invention is notlimited to this, and only pixel data having a minimum lightness datavalue or a predetermined number of pixel data having small lightnessdata values containing a minimum value may be uniquely removed.

Even if a dark spot defect is generated by a foreign matter which isdeposited on the sensing surface and detected darker (blacker) than asurrounding image pattern, abnormal pixel data in the target pixel canbe uniquely removed before the start of optimal image readingsensitivity extraction/setting operation. The image reading sensitivitycan be prevented or suppressed from being set higher than an originallyproper value. Hence, a high-quality subject image can be obtained innormal image reading operation.

When a minimum number of pixel data necessary to properly executeoptimal image reading sensitivity extraction/setting operation can beensured, a predetermined number of pixel data in descending order oflightness data values and a predetermined number of pixel data inascending order of lightness data values may be removed, as describedabove. In this case, the influence of both bright and dark spot defectscan be eliminated.

The number of specific pixel data removable by the above-describedspecific pixel data removal operation, and setting of a detection regionin sensitivity adjustment reading operation will be examined.

As described above, in specific pixel data removal operation accordingto the third embodiment, a predetermined number of (plurality of) pixeldata having maximum or minimum lightness data values are removed from apixel data group (original pixel data group) for each image readingsensitivity. To shorten the reading operation time in sensitivityadjustment reading operation, a target region, i.e., detection region 40may be set narrow, and therefore the number of pixels may be set smallwithout setting any condition or restriction. In this case, however, aplurality of pixel data are uniquely removed by specific pixel dataremoval operation, decreasing the number of processed pixel data. Thenumber of pixel data used for optimal image reading sensitivityextraction/setting operation may become insufficient, failing tonormally execute the processing.

To prevent this, the region subjected to sensitivity adjustment readingoperation must be set to have the number of pixel data enough toproperly execute optimal image reading sensitivity extraction/settingoperation even after a predetermined number of pixel data are removed inspecific pixel data removal operation.

More specifically, according to the examination by the presentinventors, the interval between the bright and dark streaks (bright anddark patterns) of the fingerprint of an adult is about 300 μm when thephotosensor system according to the third embodiment is applied to afingerprint reading apparatus. On the other hand, the interval betweenpixels (double-gate photosensors) can be manufactured as small as about50 μm in a photosensor array applied to the photosensor system accordingto the third embodiment. From this, detecting one bright/dark streak ofthe fingerprint image requires about six pixels.

Extracting an optimal image reading sensitivity with high precision inthe above-mentioned optimal image reading sensitivity extraction/settingoperation requires image data of about eight bright/dark streaks (about48 pixels). When conditions are so set as to remove about 10 pixel datain order to eliminate the influence of abnormal pixel data caused by anelement defect or deposition of a foreign matter on the sensing surfaceby the above-described specific pixel data removal operation, at least aregion of about 60 pixels, i.e., a region corresponding to about 10bright/dark streaks (corresponding to a width of about 3 mm) is set as aregion subjected to sensitivity adjustment reading operation. Even ifdata of 10 pixels are removed as pixel data having large lightness datavalues, pixel data of the number of pixels (about 50 pixels) enough toensure the extraction precision of an optimal image reading sensitivitycan be left.

In this fashion, the number of pixels (or row and column ranges)contained in the detection region in the above sensitivity adjustmentreading operation is set using, as parameters, a minimum number ofpixels necessary to ensure a sufficient extraction precision of anoptimal image reading sensitivity on the basis of an image patternserving as a subject in optimal image reading sensitivityextraction/setting operation, and the number of specific pixel data tobe removed based on the frequency of element defects generated in thephotosensor array or the size of a foreign matter deposited on thesensing surface.

Note that this examination has exhibited merely an example when thephotosensor system according to the third embodiment is applied to afingerprint reading apparatus. The present invention is not limited tothis, and the detection region is appropriately set in accordance withthe image pattern of a subject, the manufacturing precision of thephotosensor, the use environment of the photosensor system, or the like.

The effectiveness in an application of the photosensor system accordingto the present invention to a fingerprint reading apparatus will beexplained by showing some experimental data in comparison with aconventional method. Optimal image reading sensitivityextraction/setting operation (steps S46 to S49) will also be describedby referring to sensitivity setting operation of the photosensor systemaccording to the above-described embodiment.

FIGS. 17A and 17B show an example of the image pattern of a fingerprintwhen a subject image (sensitivity adjustment image) is read byperforming sensitivity adjustment reading operation in a predetermineddetection region of the photosensor array including an abnormal pixelcaused by an element defect or the like in the photosensor systemaccording to the third embodiment. FIGS. 18A to 18E are graphs showingchanges in pixel data (lightness data) for respective rows of afingerprint image obtained by sensitivity adjustment reading operation.FIGS. 19A and 19B are a graph showing changes in dynamic range based onpixel data (lightness data) obtained from the fingerprint image bysensitivity adjustment reading operation, and a correspondence tablebetween the row number, the dynamic range, and the image readingsensitivity (charge accumulating period).

FIGS. 20A and 20B show examples of the image pattern of a fingerprintwhen a fingerprint image is read by setting an image reading sensitivityusing the photosensor array including an abnormal pixel without applyingthe sensitivity setting method according to the third embodiment, andwhen the fingerprint image is read by setting an image readingsensitivity using the sensitivity setting method according to the thirdembodiment. FIGS. 21A to 21E are graphs showing changes in pixel data(lightness data) for respective rows of fingerprint image data obtainedby specific pixel data removal operation according to this embodiment.FIGS. 22A and 22B are a graph showing changes in dynamic range based onpixel data (lightness data) obtained by specific pixel data removaloperation according to the third embodiment, and a correspondence tablebetween the row number, the dynamic range, and the image readingsensitivity (charge accumulating period).

As shown in FIG. 17A, the image pattern (fingerprint) of a finger FGserving as a subject in normal image reading operation is used as asensitivity adjustment image. Different charge accumulating periods areset for photosensors on respective rows of the detection region 40 whichis defined by a row range (128 rows) of 64th to 191st rows and a columnrange (64 columns) of 67th to 130th columns, and is set in advancewithin the effective reading region 30 of the photosensor array 100 madeup of at least 256×196 pixels (photosensors). Then, sensitivityadjustment reading operation of reading the image pattern of the fingerFG is executed to obtain a fingerprint image in which the image readingsensitivity changes stepwise every row in image reading operation of oneframe, as shown in FIG. 17B. In this case, assume that an element defect(not shown) exists in the photosensor array 100 shown in FIG. 17A, andthat a bright point defect IL which corresponds to the element defectand represents an extremely high lightness data value is generated inthe fingerprint image within the detection region 40, as shown in FIG.17B.

In this fingerprint image, the lightness data value is expressed by 256gray levels. FIGS. 18A to 18E show the change trends of lightness datavalues in respective pixels in the 80th, 104th, 128th, 152nd, and 176throws for descriptive convenience. For example, as shown in FIG. 18Billustrating changes in lightness data value in the 104th row, pixeldata having an extremely high lightness data value is observed at acolumn number Rp in the 104th row corresponding to a position where thebright point defect IL shown in FIG. 17B occurs.

In the conventional sensitivity setting method described above, maximumand minimum lightness data values are extracted every row regardless ofthe presence/absence of an abnormal pixel (bright point defect IL) asshown in FIGS. 17B and 18B. The dynamic range is calculated from thedifference between the maximum and minimum values. The lightness datavalue of the abnormal pixel saturates to represent the maximum valueowing to the bright point defect IL present in the 104th row. As shownin FIGS. 19A and 19B, an apparent maximum value MA2 appears in the 104throw different from a column number RCa at which the dynamic rangeexhibits an original maximum value MA1. An image reading sensitivity(i.e., charge accumulating period T₁₀₄ of the photosensor) set for the104th row representing the maximum value MA2 is determined to be anoptimal image reading sensitivity. When the image reading sensitivityfor the 104th row is set lower than an optimal image readingsensitivity, as shown in FIG. 17B, normal image reading operation at theerroneously set image reading sensitivity results in a dark, low-qualitysubject image, as shown in FIG. 20A. Fingerprint collation processing ofcomparing the read fingerprint image with a pre-registered fingerprintimage and authenticating the fingerprint fails, and the collationprecision decreases.

To the contrary, in the photosensor system and sensitivity settingmethod according to the present invention, if the bright point defect ILoccurs in a fingerprint image due to an element defect present in thephotosensor array, as shown in FIG. 17B, the above-described specificpixel data removal operation uniquely removes one or a plurality of (onepixel data in FIGS. 21A to 21E; the position of removed pixel data isindicated by an arrow in each row) pixel data from a pixel data groupfor each image reading sensitivity (each row), as shown in FIGS. 21A to21E. Hence, at least pixel data at a column number Rp where thelightness data value is observed extremely high is necessarily removed,as shown in FIG. 21B.

Specific pixel data removal operation according to the present inventionremoves pixel data having a maximum lightness data value even in a rowfree from any abnormal pixel (bright point defect IL), as shown in FIGS.21A, 21C to 21E. Even in this case, the change trend of the dynamicrange in each row does not greatly change, as shown in FIG. 22A. The rownumber RCa at which the dynamic range exhibits the original maximumvalue MA1 can be accurately specified. An image reading sensitivity(charge accumulating period T₁₇₆) set for a corresponding row (e.g.,176th row) can be extracted and set as an optimal image readingsensitivity, as shown in FIG. 21B.

If normal image reading operation is executed at this optimal imagereading sensitivity, a high-quality subject image can be obtained with agood bright/dark contrast of the image pattern as a whole, as shown inFIG. 20B. Errors in fingerprint collation processing can be prevented orsuppressed. This means that an appropriate image reading sensitivity canbe set even in the presence of a given number of element defects in thephotosensor array or deposition of dirt on the sensing surface. In otherwords, the present invention can provide an effective technique capableof increasing the yield of a device such as a photosensor system orsensor array and simplifying maintenance or the like.

Similar to the dynamic range shown in FIG. 22A, the change trend of thedynamic range in each row subjected to optimal reading sensitivityextraction operation does not greatly change even when a plurality ofpixel data are removed in descending order of lightness data values inspecific pixel data removal operation. Similar to the above-mentionedexample, the row number RCa at which the dynamic range exhibits amaximum value can be accurately specified, and an optimal image readingsensitivity can be set.

A photosensor array drive control method in each of the aboveembodiments will be described with reference to the several views of theaccompanying drawing.

FIGS. 23A to 23J are timing charts showing an example of the drivecontrol method applicable to image reading operation of the photosensorsystem. The drive control method will be explained by referring to thearrangement of the photosensor system shown in FIG. 27, as needed.

As shown in FIGS. 23A to 23J, reset pulses φT1, φT2, . . . , φTk, φTk+1,. . . (k is a positive integer; k=1, 2, . . . , n-1) are sequentiallyapplied from a top gate driver 110 via top gate lines 101. A resetperiod T_(rst) starts, and double-gate photosensors 10 in respectiverows are initialized.

Then, the reset pulses φT1, φT2, . . . , φTk, φTk+1, . . . sequentiallyfall, the reset period T_(rst) ends, and a charge accumulating period Tastarts. Charges (holes) are generated in accordance with the amount oflight incident on the double-gate photosensors 10 in respective rows,and accumulated in the channel regions. As shown in FIG. 23I, apre-charge signal φpg is applied to a drain driver 130 to start apre-charge period T_(prch) in parallel to the charge accumulating periodTa. A pre-charge voltage Vpg is applied to drain lines 103 to performpre-charge operation of causing the drain electrodes of the double-gatephotosensor 10 in respective rows to hold predetermined voltages.

Readout pulses φB1, φB2, . . . , φBk, φBk+1, . . . are sequentiallyapplied from a bottom gate driver 120 to double-gate photosensors 10where the charge accumulating period Ta and pre-charge period T_(prch)end via bottom gate lines 102 for respective rows at timings which donot temporarily overlap the application timings of signals for resetoperation, pre-charge operation, and readout operation in other rows.Then, a readout period T_(read) starts, and changes in drain voltagesVD1, VD2, VD3, . . . , VDm corresponding to charges accumulated in thedouble-gate photosensors 10 in respective rows are simultaneouslydetected by the drain driver 130 via the drain lines 103 and read out asan output voltage Vout of serial or parallel data.

In the drive control method, the application timing interval (Tdly)between the reset pulses φT1, φT2, φT3, . . . , the readout pulses φB1,φB2, φB3, . . . , and the pre-charge signal φpg for respective rows isset equal to or longer than a total time of the reset period T_(rst) ofthe reset pulses, the readout period T_(read) of the readout pulses, andthe pre-charge period T_(prch) of the pre-charge signal:Tdly=Trst+Tprch+Tread   (1)

This setting inhibits executing reset operation, pre-charge operation,and readout operation at temporarily overlapping timings. Further, partof processing cycles in respective rows can be made to overlap eachother. Reading operation can start before reset operation in all therows ends. While two-dimensional image reading operation is properlyexecuted, the processing time of reading operation can be greatlyshortened.

FIGS. 24A to 24J are timing charts showing the first example of an imagereading sensitivity (charge accumulating period) setting method whichcan be preferably applied to sensitivity adjustment reading operationaccording to the present invention.

As shown in FIGS. 24A to 24J, the reset pulses φT1, φT2, . . . , φTn aresimultaneously applied to the top gate lines 101 each of which connectstop gate terminals TG of the double-gate photosensors 10 in the rowdirection. The reset period T_(rst) starts simultaneously on the topgate lines 101, and the double-gate photosensors 10 in respective rowsare initialized.

Then, the reset pulses φT1, φT2, . . . , φTn simultaneously fall, thereset period T_(rst) ends, and the charge accumulating periods T₁, T₂, .. . , T_(n-1), and T_(n) of the double-gate photosensors 10 in all therows simultaneously start. Charges (holes) are generated in accordancewith the amount of light incident from the top gate electrode side onthe double-gate photosensors 10 in respective rows, and accumulated inthe channel regions.

The pre-charge signal φpg and the readout pulses φB1, φB2, . . . , φBnare applied such that charge accumulating periods T₁, T₂, . . . ,T_(n-1), and T_(n) set for respective rows change stepwise every row bya predetermined delay time T_(delay), as shown in FIGS. 24A to 24J.

Accordingly, image data read at different image reading sensitivitiesfor respective rows which constitute a subject image, i.e., differentimage reading sensitivities corresponding to the number of rows can beacquired by reading a subject image (one frame) once in sensitivityadjustment reading operation.

FIGS. 25A to 25H are timing charts showing the second example of theimage reading sensitivity (charge accumulating period) setting methodwhich can be preferably applied to sensitivity adjustment readingoperation according to the present invention.

In the image reading sensitivity setting method according to thisexample, as shown in FIGS. 25A to 25H, the reset pulses φT1, φT2, . . ., φTn are sequentially applied at the time interval of a predetermineddelay time T_(delay) to the top gate lines 101 each of which connectsthe top gate terminals TG of the double-gate photosensors 10 in the rowdirection. The reset period T_(rst) starts, and the double-gatephotosensors 10 in respective rows are initialized.

Then, the reset pulses φT1, φT2, . . . , φTn fall, the reset periodT_(rst) ends, and charge accumulating periods TA₁, TA₂, . . . ,TA_(n-1), and TA_(n) sequentially start. Charges (holes) are generatedin accordance with the amount of light incident from the top gateelectrode side on the double-gate photosensors 10 in respective rows,and accumulated in the channel regions.

The pre-charge signal φpg and the readout pulses φBn, φBn-1, . . . ,φB2, and φB1 are applied such that the charge accumulating periods TA₁,TA₂, . . . , TA_(n-1), and TA_(n) set for respective rows changestepwise every row by the predetermined delay time T_(delay) after thefinal reset pulse φTn falls, as shown in FIGS. 25A to 25J.

By this sensitivity adjustment reading operation, the chargeaccumulating periods TA₁, TA₂, . . . , TA_(n-1), and TA_(n) set forrespective rows increase at a time interval twice the predetermineddelay time T_(delay). Image data read at image reading sensitivities setwith a sensitivity adjustment width of several rows or more can beacquired by reading operation of one frame.

Note that the image reading sensitivity (charge accumulating period)setting method applied to sensitivity adjustment reading operationaccording to the present invention is not limited to the above-mentionedexamples. For example, image data of a subject image may be acquired atdifferent image reading sensitivities by serially repeating a processingcycle: reset operation→charge accumulating operation→pre-chargeoperation→reading operation while sequentially changing the imagereading sensitivity. Alternatively, still another method can be adopted.

The photosensor which constitutes a photosensor array is a double-gatephotosensor in the above-described embodiments and examples, but thepresent invention is not limited to this. As far as the photosensorwhich constitutes a photosensor array performs image reading sensitivityadjustment operation, the arrangement of the sensitivity adjustmentapparatus of the photosensor system and the sensitivity adjustmentmethod according to the present invention can be preferably applied.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A photosensor system comprising: a photosensor array comprising aplurality of photosensors arrayed two-dimensionally; an image readingsection which reads a subject image at a predetermined image readingsensitivity, corresponding to a charge accumulating period, by using thephotosensor array; sensitivity adjustment image reading means forreading a sensitivity adjustment subject image using a plurality ofimage reading sensitivities; optimal image reading sensitivity derivingmeans for deriving an image reading sensitivity optimal for reading thesubject image, based on a pixel data group relating to an image patternof the subject image read by the sensitivity adjustment image readingmeans; wherein the optimal image reading sensitivity deriving meanscomprises specific data removal means for removing from the pixel datagroup specific pixel data including at least pixel data having a maximumor minimum value in the pixel data group so as to correct the pixel datagroup, wherein the optimal image reading sensitivity is derived based onthe corrected pixel data group; image reading sensitivity setting meansfor setting the optimal image reading sensitivity as the image readingsensitivity in the image reading section; and reading control means forcontrolling execution of reading of the subject image, by using theimage reading sensitivity set by the image reading sensitivity settingmeans.
 2. A system according to claim 1, wherein: the optimal imagereading sensitivity deriving means further comprises image readingsensitivity correction means for correcting only one time a value of theimage reading sensitivity optimal for reading the subject image derivedbased on the pixel data group so that the value increases by a valuecorresponding to a predetermined period-increase rate so as to correctthe image reading sensitivity, wherein the corrected image readingsensitivity is set as the optimal image reading sensitivity; and theimage reading sensitivity correction means comprises means forcorrecting the charge accumulating period corresponding to the imagereading sensitivity optimal for reading the subject image to increasethe charge accumulating period by a period corresponding to thepredetermined period-increase rate.
 3. A system according to claim 1,wherein: the optimal image reading sensitivity deriving means furthercomprises image reading sensitivity correction means for correcting onlyone time a value of the image reading sensitivity optimal for readingthe subject image derived based on the pixel data group so that thevalue increases by a value corresponding to a predeterminedperiod-increase rate so as to correct the image reading sensitivity,wherein the corrected image reading sensitivity is set as the optimalimage reading sensitivity; the optimal image reading sensitivityderiving means comprises standard image reading sensitivity extractionmeans for extracting a standard image reading sensitivity as the imagereading sensitivity optimal for reading the subject image, based on apixel data group for each of the image reading sensitivities with whichthe sensitivity adjustment subject image is read by the sensitivityadjustment image reading means; and the image reading sensitivitycorrection means corrects the value of the standard image readingsensitivity so that the value is increased by the predeterminedperiod-increase rate.
 4. A system according to claim 3, wherein thestandard image reading sensitivity extraction means comprises: dataextraction means for extracting maximum and minimum values from thepixel data group for each said image reading sensitivity; data rangecalculation means for calculating a data range of the pixel data groupfor each said image reading sensitivity based on the maximum and minimumvalues of the pixel data group, and extraction means for extracting thestandard image reading sensitivity based on the data range for each saidimage reading sensitivity.
 5. A system according to claim 4, wherein theextraction means extracts as the standard image reading sensitivity animage reading sensitivity having a maximum data range out of data rangesfor the respective image reading sensitivities.
 6. A system according toclaim 1, wherein the optimal image reading sensitivity deriving meansfurther comprises image reading sensitivity correction means forcorrecting only one time a value of the image reading sensitivityoptimal for reading the subject image derived based on the pixel datagroup so that the value increases by a value corresponding to apredetermined period-increase rate so as to correct the image readingsensitivity, wherein the corrected image reading sensitivity is set asthe optimal image reading sensitivity, and wherein the period-increaserate is arbitrarily set from outside the photosensor system.
 7. systemaccording to claim 3, wherein the optimal image reading sensitivityderiving means comprises means for comparing a mean value of the pixeldata group at the standard image reading sensitivity with apredetermined reference value, and judges whether correction based onthe predetermined period-increase rate should be executed.
 8. A systemaccording to claim 7, wherein the reference value corresponds to amedian value of a data range of the pixel data group at the standardimage reading sensitivity.
 9. A system according to claim 7, wherein thereference value is arbitrarily set from outside the photosensor system.10. A system according to claim 1, wherein the optimal image readingsensitivity deriving means comprises: image reading sensitivityextraction means for extracting an image reading sensitivity suitablefor a normal reading operation of the subject image based on the pixeldata group from which the specific pixel data has been removed by thespecific data removal means, and setting means for setting as theoptimal image reading sensitivity the image reading sensitivityextracted by the image reading sensitivity extraction means.
 11. Asystem according to claim 10, wherein the image reading sensitivityextraction means comprises: data extraction means for extracting maximumand minimum values from the pixel data group from which the specificdata has been removed for each said image reading sensitivity, datarange calculation means for calculating a data range of the pixel datagroup for each said image reading sensitivity based on the maximum andminimum values of the pixel data group extracted by the data extractionmeans, and extraction means for extracting an image reading sensitivityhaving a maximum data range out of data ranges of pixel data groups forthe respective image reading sensitivities.
 12. A system according toclaim 1, wherein the specific data removal means removes a plurality ofpixel data sequentially from a maximum value from the pixel data groupobtained by the sensitivity adjustment image reading means for each saidimage reading sensitivity.
 13. A system according to claim 1, whereinthe specific data removal means removes a plurality of pixel datasequentially from a minimum value from the pixel data group obtained bythe sensitivity adjustment image reading means for each said imagereading sensitivity.
 14. A system according to claim 1, wherein thesensitivity adjustment image reading means reads the sensitivityadjustment subject image only in a detection region set in advancewithin an effective reading region of the photosensor array.
 15. Asystem according to claim 14, wherein the detection region is set to bea region where the pixel data group from which the specific pixel datahas been removed by the specific data removal means contains a minimumnumber of pixel data necessary to normally execute at least a readingsensitivity setting operation of the image reading sensitivity settingmeans.
 16. A system according to claim 1, wherein a reading operation ofthe subject image in the sensitivity adjustment image reading means isexecuted by setting the image reading sensitivities to differ stepwisefor a predetermined number of rows in the photosensor array.
 17. Asystem according to claim 1, wherein the sensitivity adjustment imagereading means reads the sensitivity adjustment subject image within anentire effective reading region of the photosensor array.
 18. A systemaccording to claim 1, wherein the pixel data group includes lightnessdata corresponding to the image pattern of the subject image.
 19. Asystem according to claim 1, wherein each photosensor comprises sourceand drain electrodes which are formed on respective sides of a channellayer formed from a semiconductor layer, and first and second gateelectrodes which are formed above and below the channel region viainsulating films, wherein one of the first and second gate electrodes isset as a light irradiation side, and charges corresponding to an amountof light incident onto the light irradiation side are generated andaccumulated in the channel region.
 20. A system according to claim 1,wherein the optimal image reading sensitivity deriving means furthercomprises image reading sensitivity correction means for correcting onlyone time a value of the image reading sensitivity optimal for readingthe subject image derived based on the pixel data group so that thevalue increases by a value corresponding to a predeterminedperiod-increase rate having a predetermined constant value so as tocorrect the image reading sensitivity, wherein the corrected imagereading sensitivity is set as the optimal image reading sensitivity. 21.A drive control method for a photosensor system which includes aphotosensor array comprising a plurality of two-dimensionally arrayedphotosensors, and which reads a subject image by using the photosensorarray, said method comprising: reading a sensitivity adjustment subjectimage using a plurality of image reading sensitivities corresponding torespective charge accumulating periods; deriving an image readingsensitivity suitable for reading the subject image based on a pixel datagroup relating to an image pattern of the subject image read from thesensitivity adjustment subject image; wherein deriving the image readingsensitivity comprises removing from the pixel data group specific pixeldata including at least pixel data having a maximum or minimum value inthe pixel data group so as to correct the pixel data group; and settingthe derived image reading sensitivity as a reading sensitivity forreading the subject image thereby executing reading of the subjectimage, by using the image reading sensitivity.
 22. A method according toclaim 21, wherein: deriving the image reading sensitivity furthercomprises correcting only one time a value of the image readingsensitivity suitable for reading the subject image derived based on thepixel data group so that the value increases by a value corresponding toa predetermined period-increase rate so as to correct the image readingsensitivity; and correcting the value of the image reading sensitivitysuitable for reading the subject image comprises correcting the chargeaccumulating period corresponding to the image reading sensitivity so asto increase the charge accumulating period by a period corresponding tothe predetermined period-increase rate.
 23. A method according to claim21, wherein: deriving the image reading sensitivity further comprisescorrecting only one time a value of the image reading sensitivitysuitable for reading the subject image derived based on the pixel datagroup so that the value increases by a value corresponding to apredetermined period-increase rate so as to correct the image readingsensitivity; deriving the image reading sensitivity suitable for readingthe subject image comprises extracting a standard image readingsensitivity as the image reading sensitivity suitable for reading thesubject image, based on a pixel data group for each of the image readingsensitivities used in reading the sensitivity adjustment subject image;and correcting the value of the image reading sensitivity suitable forreading the subject image comprises correcting the value of the standardimage reading sensitivity so that the value is increased by thepredetermined period-increase rate.
 24. A method according to claim 23,wherein extracting the standard image reading sensitivity comprises:extracting maximum and minimum values from the pixel data group for eachsaid image reading sensitivity; calculating a data range of the pixeldata group for each said image reading sensitivity based on the maximumand minimum values of the pixel data group, and extracting the standardimage reading sensitivity based on the data range for each said imagereading sensitivity.
 25. A method according to claim 24, whereinextracting the standard image reading sensitivity based on the dataranges comprises comparing data ranges for the respective image readingsensitivities, and extracting as the standard image reading sensitivityan image reading sensitivity having a maximum data range.
 26. A methodaccording to claim 21, wherein deriving the image reading sensitivityfurther comprises correcting only one time a value of the image readingsensitivity suitable for reading the subject image derived based on thepixel data group so that the value increases by a value corresponding toa predetermined period-increase rate so as to correct the image readingsensitivity, and the period-increase rate is arbitrarily set fromoutside the photosensor system.
 27. A method according to claim 23,wherein deriving the image reading sensitivity suitable for reading thesubject image comprises comparing a mean value of the pixel data groupat the standard image reading sensitivity with a predetermined referencevalue, and judging whether correction based on the predeterminedperiod-increase rate should be executed.
 28. A method according to claim27, wherein the reference value corresponds to a median value of a datarange of the pixel data at the standard image reading sensitivity.
 29. Amethod according to claim 27, wherein the reference value is arbitrarilyset from outside the photosensor system.
 30. A method according to claim21, wherein deriving the image reading sensitivity suitable for readingthe subject image comprises extracting an image reading sensitivitysuitable for a normal reading operation of the subject image based onthe pixel data group from which the specific pixel data has beenremoved.
 31. A method according to claim 30, wherein extracting theimage reading sensitivity suitable for the normal reading operation ofthe subject image comprises: extracting maximum and minimum values fromthe pixel data group from which the specific data has been removed foreach said image reading sensitivity, calculating a data range of thepixel data group for each said image reading sensitivity based on themaximum and minimum values of the pixel data group extracted for eachsaid image reading sensitivity, and extracting an image readingsensitivity having a maximum data range out of data ranges of pixel datagroups for the respective image reading sensitivities.
 32. A methodaccording to claim 21, wherein removing the specific pixel datacomprises: comparing sizes of all pixel data contained in the pixel datagroup for each said image reading sensitivity, and sorting the pixeldata; and removing a predetermined number of pixel data from at leastone end of a string of the sorted pixel data.
 33. A method according toclaim 21, wherein removing the specific pixel data comprises repeating apredetermined number of times: comparing sizes of all pixel datacontained in the pixel data group for each said image readingsensitivity, and extracting and removing pixel data having a maximum orminimum value.
 34. A method according to claim 21, wherein reading thesensitivity adjustment subject image comprises setting the image readingsensitivities to differ stepwise for a predetermined number of rows inthe photosensor array.
 35. A method according to claim 21, wherein thepixel data group includes lightness data corresponding to the imagepattern of the subject image.
 36. A method according to claim 21,wherein each photosensor comprises source and drain electrodes which areformed on respective sides of a channel layer formed from asemiconductor layer, and first and second gate electrodes which areformed at least above and below the channel region via insulating films,wherein one of the first and second gate electrodes is set as a lightirradiation side, and charges corresponding to an amount of lightentering from the light irradiation side are generated and accumulatedin the channel region.
 37. A method according to claim 21, whereinderiving the image reading sensitivity further comprises correcting onlyone time a value of the image reading sensitivity suitable for readingthe subject image derived based on the pixel data group so that thevalue increases by a value corresponding to a predeterminedperiod-increase rate having a predetermined constant value so as tocorrect the image reading sensitivity.